<Dispersing Agent>
It is known that, when producing an ink or the like, it is difficult to stably disperse high concentrations of pigments and many problems are caused in the manufacturing steps and products. For example, many products obtained by dispersing fine particles of pigments have a high viscosity and is difficult to remove them from a dispersing or transportation apparatus. When the dispersed product has an extremely high viscosity, it turns into a gel during storage and becomes difficult to use. Further, the glaze is lowered and undesirable conditions such as a bad leveling occur on the surface that the product is applied to. When a mixture of different pigments is used, a color separation by aggregation, a coloring unevenness or a considerable coloring reduction by sedimentation or the like may occur on the surface that the product is applied to.
Thus, a dispersing agent is generally used to consistently maintain a dispersed state. The dispersing agent has a structure containing a site for adsorbing a pigment and a site having a high affinity to the solvent which is the dispersion medium. Properties of the dispersing agent are dependent on a balance of the two functional sites. Various dispersing agents are used in accordance with surface conditions of the pigment to be dispersed. A basic dispersing agent is generally used for a pigment having an acidic surface. In this case, a basic functional group is the site for adsorbing the pigment. Dispersing agents having an amino group as the basic functional group are disclosed in, for example, Patent Documents 1 to 6.
The dispersing agents disclosed in Patent Documents 1 to 3 have dispersibility. However, the side chains thereof are limited, and thus, solvents or binder resins which may be used are limited. The dispersing agents disclosed in Patent Documents 4 to 6 have a certain degree of dispersibility. However, they have to be used in large amounts to obtain a stably dispersed product having a low viscosity. However, it is preferable not to use a large amount of the dispersing agent, in view of a possible reduction of a coating durability when applied to an ink, a coating composition, or the like.
Patent Documents 7 and 8 disclose a polyurethane resin characterized in that a polyol, prepared by polymerizing a compound having a polymerizable unsaturated bond with a mercaptan compound having two hydroxy groups as an initiator, is used. However, the polyurethane resin does not contain a residual amine as a basic site, and thus does not exhibit a sufficient dispersibility to a pigment having an acidic surface, when the polyurethane resin is used as a dispersing agent.
Further, Patent Document 9 discloses a polyurethane urea resin characterized in that a polyol, prepared by polymerizing a compound having a polymerizable unsaturated bond with a mercaptan compound having two hydroxy groups as an initiator, is used. However, the polyurethane urea resin is a particulate polyurethane, and thus does not exhibit a sufficient dispersibility to a pigment having an acidic surface, when the polyurethane urea resin is used as a dispersing agent.
<Inkjet Ink>
In the past, an ink prepared by dissolving a water-soluble dye, such as an acid dye, a direct dye, or a basic dye, in a glycolether-based solvent and water was widely used as an inkjet ink. Recently, however, an ink containing a pigment is prevailingly used as an ink in the field of advertising signs requiring durability. However, it is known that, when the ink is prepared, it is difficult to stably disperse pigments at high concentration, and many problems are caused in the manufacturing steps and products. For example, many dispersed products of fine particles of pigments have a high viscosity. In case of an extremely high viscosity, it may turn into a gel during storage and become difficult to use. A high stability is required for an inkjet ink, in particular, because the ink is discharged from a fine nozzle. Thus, a viscosity increase during storage may become a fatal defect because a nozzle may be clogged and inhibit discharging the ink.
Thus, a dispersing agent is generally used to consistently maintain a dispersed state. In particular, a dispersing agent having an amino group as a basic functional group is preferentially used, and is disclosed in, for example, Patent Documents 10 and 11.
However, the dispersing agents disclosed in Patent Documents 10 and 11 have a problem of a low coating durability, although they have a high dispersion stability. Further, when the dispersing agents disclosed in Patent Documents 10 to 12 are used, the binder resins which can be combined therewith are limited. Thus, these dispersing agents are not preferable because a coating durability, and/or a print stability may be lowered.    [patent reference 1] Japanese Unexamined Patent Publication (Kokai) No. 9-169821    [patent reference 2] Japanese Unexamined Patent Publication (Kokai) No. 9-194585    [patent reference 3] Japanese Unexamined Patent Publication (Kokai) No. 2004-089787    [patent reference 4] Japanese Unexamined Patent Publication (Kokai) No. 1-236930    [patent reference 5] Japanese Unexamined Patent Publication (Kokai) No. 3-103478    [patent reference 6] Japanese Unexamined Patent Publication (Kokai) No. 2004-344795    [patent reference 7] Japanese Translation Publication (Kohyo) No. 57-500246    [patent reference 8] Japanese Unexamined Patent Publication (Kokai) No. 63-066211    [patent reference 9] Japanese Unexamined Patent Publication (Kokai) No. 5-194702    [patent reference 10] Japanese Unexamined Patent Publication (Kokai) No. 2007-284642    [patent reference 11] Japanese Unexamined Patent Publication (Kokai) No. 2006-056990    [patent reference 12] Japanese Unexamined Patent Publication (Kokai) No. 2007-169492